As we know a solenoid is considered as a electromagnet(magnet) if there's a current flowing through it. if a soft iron core is placed inside the solenoid, the former get magnetised. Consider the solenoid as a hollow bar magnet, and the magnetised soft iron core inside as a bar magnet. The poles of the magnets next to each other is of same polarity. Is there attracting force between the two magnets? If so, does it mean "like poles attract, unlike poles repel" while two magnets are placed such that one is inside another?
[Physics] Is it correct to say “like poles attract, unlike poles repel” while two magnets are placed such that one is inside another
electromagnetism
Best Answer
A solenoid induces a magnetic field in the iron bar kept inside it. This is not the same as inserting a permanent bar magnet inside the solenoid.
The induced magnetic field is in the same direction as the original magnetic field. One way of thinking about this is that the domains in the iron bar line up with the external field, producing a net magnetic field in the same direction as the original.
However if you consider a permanent bar magnet kept in a solenoid, and the solenoid was large enough such that the bar magnet could rotate in any arbitrary direction, once the solenoid is switched on, the bar magnet will align itself with the solenoid's field. This has to do with minimizing the energy in the magnetic field. The energy is given by $$ E = - m .\vec{B} $$
so clearly if $m$ and $B$ are in the same direction, it is a lower energy configuration than if $m$ and $B$ are in opposite directions.